1. Field of the Invention
The present invention generally relates to measurement systems configured to perform measurements of a specimen and illumination subsystems configured to provide illumination for a measurement system. Certain embodiments relate to a measurement system in which VUV and non-VUV light is directed along a common optical path.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Optical systems play a significant role in the manufacturing of integrated circuits and other semiconductor devices. For example, optical metrology and/or inspection tools are used for quality control purposes in semiconductor manufacturing. The capability and throughput of these optical systems can have a significant impact on semiconductor manufacturing. For example, the throughput of an optical metrology and/or inspection tool has a direct impact on the throughput of a semiconductor manufacturing process (e.g., as the throughput of the tool decreases, the throughput of the process decreases). Likewise, the resolution capability of an optical metrology and/or inspection tool can have a significant impact on a semiconductor manufacturing process since the accuracy of the optical metrology and/or inspection tool can directly affect how well the process is controlled.
The resolution of an optical system depends to a large extent on the wavelength of the optical system as well as other parameters such as numerical aperture (NA). For example, as the wavelength of the optical system is decreased, the optical system can image features having smaller and smaller dimensions thereby increasing the resolution of the system. Some metrology and/or inspection tools used in semiconductor manufacturing today are designed for use with light having a wavelength of 248 nm. However, metrology and/or inspection tools that are designed for use with light having a wavelength of 193 nm or shorter (for example, 157 nm) are becoming more common in semiconductor research and manufacturing.
Metrology and/or inspection tools may be designed for such wavelengths since lithography tools are also designed for these wavelengths. For example, such wavelengths are sometimes used to examine materials such as resists at the wavelengths to which they will be exposed during a lithography process. In addition, a great deal of information about these wavelengths of light and the issues surrounding their implementation in optical tools is generated during the lithography tool design process that can be used to aid in the design of a metrology and/or inspection tool. Optical metrology and/or inspection tools that operate at vacuum ultraviolet (VUV) wavelengths are also being developed for semiconductor research and manufacturing as these wavelengths of light will be used in future lithography processes.
Spectroscopic measurements are also becoming more and more important in many areas, especially in semiconductor industries. To meet the next generation tool requirements, measurements across a broader spectral range must be provided. It is greatly desirable to use a spectrum that is as broad as possible with a single light source. However, it is difficult to find a single light source covering a broad enough spectrum, for example, from VUV to infrared (IR). Most of the time, it is necessary to use several light sources to cover the required broadband spectrum.
Several different ways of coupling different light sources into a common optical path are currently being used. For example, one previous approach is to use a “see-thru” lamp. This technique combines a deuterium lamp with a tungsten lamp. One of the lamps is imaged into the other lamp which has a see-thru configuration. This technique only applies to see-thru lamps and adds complexity to the optical subsystem in that additional imaging optics are required.
Another technique for combining light beams from different light sources is to use a flip-in mirror. However, a flip-in mirror will create residual polarization in the deflected beam. As such, this technique cannot be used in some forms of ellipsometry or reflectometry applications because the residual polarization due to the reflection on the flip-in mirror will distort the measurements. It is possible to correct the residual polarization effect; however, such corrections require additional calibration procedures and degrade the accuracy of the measurement system.
A different technique for increasing the spectroscopic capability of a measurement system is to use two or more separate optical subsystems combined into one measurement system. However, combining separate optical subsystems into a measurement system greatly increases the cost of the measurement system. In addition, multiple optical subsystems require additional calibrations and increase the complexity of the measurement system.
Two different measurement systems, each configured to perform measurements in different wavelength regimes, may also be used to measure a specimen such that measurements across a much broader range of wavelengths may be achieved. However, using a combination of measurement systems increases the cost of hardware and software. In addition, the use of a combination of measurements from different systems to obtain information on one specimen makes the calibration and matching of the different systems complex and difficult.
Accordingly, it would be advantageous to develop illumination subsystems that can provide illumination for a measurement system across a relatively wide range of wavelengths (e.g., from VUV to IR) without introducing residual polarization to the light, without substantially increasing the complexity or cost of the measurement system, and without requiring additional calibrations while maintaining the accuracy of the measurement system.